Abnormal dispersion glass exhibiting the abnormal partial dispersion in the infrared region

ABSTRACT

THIS INVENTION PROVIDES AN ABNORMAL DISPERSION GLASS EXHIBITING THE ABNORMAL PARTIAL DISPERSION IN THE INFRARED REGION. THIS ABNORMAL DISPERSION GLASS HAS A SMALL ABBE NUMBER AND FAVORABLE ACID RESISTANCE, AND IS COMPOSED OF B2O3 54 TO 60 MOL. PERCENT, PBO 6 TO 12 MOL. PERCENT AL2O3 8 TO 14 MOL. PERCENT COA 8 TO 15 MOL. PERCENT ZRO2 0 TO 1.5 MOL. PERCENT, TIO2 1.5 TO 4.0 MOL. PERCENT AND SIO2 4 TO 11 MOL. PERCENT.

p 1974 YOSHIYUKI A-SAHARA ETAL 3,834,913

ABNORMAL DISPERSION GLASS EXHIBITING THE ABNORMAL PARTIAL DISPERSION IN THE INFRARED REGION Filed July 10, 1972 United States Patent Olfice 3,834,913 Patented Sept. 10, 1974 Int. Cl. C03c 3/b4, 3/10, 3/30 US. Cl. 106-53 2 Claims ABSTRACT OF THE DISCLOSURE This invention provides an abnormal dispersion glass exhibiting the abnormal partial dispersion in the infrared region. This abnormal dispersion glass has a small Abbe number and favorable acid resistance, and is composed of B 54 to 60 mol. percent, PhD 7 to 12 mol. percent, A1 0 8 to 14 mol. percent, CaO 8 to 15 mol. percent, ZrO 0 to 1.5 mol. percent, TiO 1.5 to 4.0 mol. percent and SiO' 4 to 11 mol. percent.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an abnormal dispersion glass exhibiting the abnormal partial dispersion in the infrared region, having a small Abbe number and being quite resistant to attack of acids.

2. Description of the Prior Art In the known flint and crown glasses, there is the following linear relation (2) between the partial dispersion ratio (P and the Abbe number (v if it is assumed that the partial dispersion ratio P with respect to mutually diflerent wave lengths x and y is represented by equation (1).

wherein P is the partial dispersion ratio for the known flint and crown glass, n and n are respectively the refractive indexes with respect to lights having more length x and y, a and b are the constants changing with the magnitude of the wave length of X and Y, respectively.

In order to design and produce a super achromatic lens assembly capable of extinguishing the secondary spectrum, at least one lens having the partial dispersion ratio P, deviating from the above formula (2), must be used. This means that at least one lens whose AP is not zero must be used, assuming that the partial dispersion ratio P of the glass is shown by following relation;

The favorable super achromatic lens has a higher AP value. The special Kurtz flint glass is already known having a refractive index (n and the Abbe number (11 comparable with that of the usual flint glass, and having a large partial dispersion ratio (P to the C line and 13 line in the infrared region. This means that the AP of the Kurtz flint glass is larger than zero. For example, the usual flint glass has a P value of 0.7551 with a 11,, of 1.61340 and a 11,, of 44.30, whereas the Kurtz flint glass has a P value of 0.7956. This Kurtz flint glass,

however, has a very poor acid resistance since it is composed primarily of PbO--B O In a test for the resistance of the glass to acidic attack comprising sampling a 28-35 mesh glass powder (specific gravity gram) in a Pt cage, dipping the Pt cage for 1 hr. in a cc. of 1/100N HNO solution kept at above 99 C. using boiling water, then drying the powder, and measuring the weight loss (percent) of the glass powder, the Kurtz flint glass showed a weight loss of greater than 4.0% by weight, whereas the usual flint or crown glass shows a weight loss of less than 2.0%. This means that the Kurtz flint glass had a poor resistance to acid attack.

On the other hand, the difference between the Abbe number of the concave lens and that of the convex lens, both lenses being assembled to minimize achromatic aberration, may be as large as possible not only to remove the secondary spectrum but also to remove the spherical aberration. Therefore, the abnormal dispersion flint glass for the concave lens must have a small 11,, value and also a small Abbe number.

The inventors have already invented a PbO'-B O' Al O glass incorporating CaO or ZrO as Japanese Patent Application 69533/70, which exhibited excellent resistance to acid attack. It is an object of this invention to improve further the acid resistance of such a glass.

SUMMARY OF THE INVENTION This invention provides an abnormal dispersion glass exhibiting the abnormal partial dispersion in the infrared region, having a small Abbe number and favorable acid resistance which is composed of B 0 54 to 60 mol. percent, PbO 7 to 12 mol. percent, A1 0 8 to 14 mol. percent, CaO 8 to 15 mol. percent, ZrO 0 to 1.5 mol. percent, TiO 1.5 to 4.0 mol. percent and Si0 4 to 11 mol. percent.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING In the drawing the relationship between the 11,, value and the n value of a glass of the invention is shown.

DETAILED DESCRIPTION OF THE INVENTION This invention is based on the discovery that the introduction of both TiO 1.5-4.0 mol. percent and SiO' 4.0-- 11 mol. percent to an acid resistant glass composed of B 0 54-60 mol. percent, Pb'O 7-12 mol. percent, A1 0 8-14 mol. percent and Zr0 0-1.5 mol. percent was very effective to reduce the 11,, value, to improve the acid resistance additionally and to produce a lens suitable for removing spherical aberration.

Preferred ranges of the above components of the glass composition of this invention are; B 0 54 to 58 mol. percent, PhD 8 to 10 mol. percent, A1 0 11 to 13 mol. percent, CaO 9 to 11 mol. percent, ZrO 0.5 to 1.5 mol. percent, TiO 2.5 to 3.5 mol. percent and SiO 8 to 10%.

Now, some examples of this invention and comparative examples will be described with reference to Table 1 and Table 2, and a drawing which shows the relation between the ai value and the n value of the glasses of this invention.

TABLE 1 Composition (mol. percent) Specimen number B 03 A1 0 PbO CaO ZrOz T101 $102 *Comparative examples of conventional glass compositions.

TABLE 2 Specimen number 11a Pig 1 D shows the weight loss (percent) of the glass powder subjected to the 1/100 N HNOa solution described hcreinbetore.

2 P Shows the density (g./cc.) of the glass.

*Comparative examples of conventional glass compositions.

The above table shows the following things. The single addition of Ti to the glass of Japanese Patent Application 69533/70 improves the acid resistance to some extent surpassing that of the conventional Kurtz flint glass, but the D, value is not reduced below 2.00%, as observed in Specimen 8. The single addition of the SiO to the glass according to the Japanese Patent Application is detrimental to the improvement of the acid resistance, as observed in the Specimens 4, 5, 6 and 7.

The addition of both TiO and SiO to the glass of the Japanese Patent Application brings out a low refractive index, a low Abbe number and excellent acid resistance with a D,,, value not above 2.00, as shown in the Specimens 1 and 2, whereas the acid resistance of conventional glasses having the same refractive index and the Abbe number as specimen 1 and 2. (Specimens 3, 5, 8) are not so good.

In the drawing, the relation between the r value and the n value of the glass of this invention (Specimen 1 and 2) is shown as points 1 and 2.

The above restricted ranges of the contents of PbO B 0 A1 0 CaO and ZrO are suitable for producing a stable and acid resistant glass and impart the abnormal partial dispersion in the infrared region to the glass.

Less than 1.5% of TiO;,, is ineffective to reduce the Abbe number of the glass, whereas more than 4.0% of TiO facilitates the formation of a devitrifying glass.

More than 11.0% of SiO extinguishes the anomaly of the partial dispersion of the glass in the infrared region, whereas less than 4.0% of the SiO is almost ineffective to improve the acid resistance of the glass.

The glass having the above composition exhibits the remarkable anomaly of the partial dispersion in the infra red region but exhibits the low partial dispersion ratio P in the ultrared region as compared with the usual flint glass. For example, Specimen 2 of this invention has a P value of 0.8011, a P value of 1.3347 and a P value of 0.5639, whereas the usual flint glass having the same Abbe number has a P value of 0.7564, a P value of 0.5688 and a Pi value of 1.3504, determined by the aforesaid expression (2).

The optical glass of this convention can be prepared by melting a mixture of the above described components in a platinum crucible at a temperature of 1300 to 1400 C., refining the melt, casting the glass in a desired form and cooling to room temperature.

While this invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention.

Therefore, the appended claims are intended to cover all such equivalent variations as coming within the true spirit and scope of this invention.

What is claimed is:

1. An abnormal dispersion glass exhibiting partial dispersion in the infrared region, having a low Abbe number and an acid resistance D, value not above 2.00, and composed of B 0 54 to mol. percent, PhD 7 to 12 mol. percent, A1 0 8 to 14 mol. percent, CaO 8 to 15 mol. percent, ZrO O to 1.5 mol. percent, TiO 1.5 to 4.0 mol. percent and SiO 4 to 11 mol. percent.

2. The abnormal dispersion glass of Claim 1, wherein said glass is composed of B 0 54 to 58 mol. percent, PbO 8 to 10 mol. percent, A1 0 11 to 13 mol. percent, CaO 9 to 11 mol. percent, ZrO 0.5 to 1.5 mol. percent, TiO; 2.5 to 3.5 mol. percent, and SiO 8 to 10 mol. percent.

References Cited UNITED STATES PATENTS 11/1969 Reid et al 10653 ALLEN B. CURTIS, Primary Examiner M. L. BELL, Assistant Examiner 

